Various techniques can be used to cause a therapeutic agent to penetrate tissue. For example, injections are commonly used to deliver a therapeutic agent to a location in or below the skin. Other techniques such as iontophoresis and sonophoresis can also be used and may be less invasive and/or painful. Iontophoresis uses an electrical field to drive an ionic therapeutic agent into target tissue. Sonophoresis uses acoustic energy to increase the permeability of tissue and drive a therapeutic agent that is suspended in a glycerin coupling gel into the target tissue. Each of these techniques is most frequently used for the transdermal administration of topical therapeutic agents. Although iontophoresis and sonophoresis have certain benefits over injections, such as being less invasive and painful, they have not been widely adopted. One primary limitation of these techniques is the inability to ensure accurate dosing. With iontophoresis, a user must fill a reservoir with the required amount of therapeutic agent prior to use. Moreover, once the device is filled, conventional iontophoresis systems lack the capability to accurately determine how much of the therapeutic agent is being absorbed. Furthermore, certain tissues can lack sufficient permeability to adequately administer a complete dose. It is likewise difficult to accurately administer a dose of therapeutic agent using sonophoresis. A therapeutic agent is suspended in acoustic coupling gel, which cannot be fully absorbed into the skin. The residual coupling gel left on the skin retains an unknown quantity of suspended therapeutic agent, which makes it difficult to determine how much of the therapeutic agent is properly administered.